roll cage question

[Cobra-R]

We are in the midst of installing a cage and have been talking to Kurt, but wanted some more clarification on this. We want to run a cross brace like this between the rear shock towers (95 Cobra):

 

and bring the rear supports down and attach them into the cross bar. What is illegal about this and what can be done to make it legal?

 

First we were told it was ok, now apparently it isn't. Is there something that we can do to make make this legal without cutting everything back off?

[Guest]

What was the reason that you were told that makes it not legal?

[Tom Beverly]

I'm guessing the reason you were told it was illegal, is because the NASA powers that be want the actual rear down tube to be directly welded to the floor panel, (not to a tube running across the floor pan)... This recently came up with Lewis Tanner's CMC car, even though several AI/CMC cars are already running the "illegal" cage design.

[Cobra-R]

What was the reason that you were told that makes it not legal?

 

They want the tube in compression rather than shear.

Im my mind the above method would be stronger since it would be welded to the shock towers (a structural part of the chassis) as well as welded to the floor. Would you agree?

 

Brian

[johnbasf]

I would say Cobra R's design would be just fine. The shock towers are stronger than the floorpan.

[Guest]

What was the reason that you were told that makes it not legal?

 

They want the tube in compression rather than shear.

Im my mind the above method would be stronger since it would be welded to the shock towers (a structural part of the chassis) as well as welded to the floor. Would you agree?

 

Brian

There are plenty of cars running out there today that have the downtubes running into the shock tower brace.

[Guest]

What was the reason that you were told that makes it not legal?

 

They want the tube in compression rather than shear.

Im my mind the above method would be stronger since it would be welded to the shock towers (a structural part of the chassis) as well as welded to the floor. Would you agree?

 

Brian

 

Something you said in this is wrong. Whom ever told you that the downtubes would be in shear if welded onto this tube was wrong. The downtubes are still in compression if the main hoop were to move.

 

I think that everyone you talked to about this needs to think about this again. I can draw a nice free body diagram if you like.

[Cobra-R]

I am just repeating what I was told, my "logic engineering" says what we want to do would be safer/stronger.

 

Brian

 

What was the reason that you were told that makes it not legal?

 

They want the tube in compression rather than shear.

Im my mind the above method would be stronger since it would be welded to the shock towers (a structural part of the chassis) as well as welded to the floor. Would you agree?

 

Brian

 

Something you said in this is wrong. Whom ever told you that the downtubes would be in shear if welded onto this tube was wrong. The downtubes are still in compression if the main hoop were to move.

 

I think that everyone you talked to about this needs to think about this again. I can draw a nice free body diagram if you like.

[trackboss]

My old AI west car had a similar tube with the rear legs from the main hoop mounting to it as described above as do most all the AI west cars. The new car I am building is the same way. I have added some short tubes to effectively "continue" the rear legs to the frame. I would not be concerned about this being illegal. This is a good thing to do as far as the cage goes as long as the tube is sized properly. If I knew how to attach photos I would show you how I did mine on the new car

[mwilson7]

I'm guessing the reason you were told it was illegal, is because the NASA powers that be want the actual rear down tube to be directly welded to the floor panel, (not to a tube running across the floor pan)... This recently came up with Lewis Tanner's CMC car, even though several AI/CMC cars are already running the "illegal" cage design.

 

What rule(s) were used to determine that this was illegal? 15.6.11 doesn't specify that the rear braces need to be attached to the floor.

[onralz]

Obviously I can't attach pictures either.

[swhiteh3]

I think the reasoning here is that the cross bar IS in shear when the rear downtubes would be in compression. Let me explain.

 

If the car is on it's roof, for instance, and there is a compression loading on the rear supports, then they are pushing down on the shock bar. If the rear tubes were, let's say, attached in the middle of the shock bar, the shock bar would be in bending. As you move those mounts out to the edges, there is less bending load on the shock bar, but the welds at the ends are still in shear - not necessarily a critical flaw, but certainly an opportunity for improvement.

 

When I look at that HANS product above, my eyes are attracted to the large gap between the floor plate and the bottom of the bar. They probably do this so a street car can fit the carpet in between. If the bar were to be lower, say sitting right on top of the horizonatal floor doubler plate and seam welded to it along it's length, then I think you've resolved the issue, since compression loading in the rear down tubes now has a direct load path into the floor doubler panels.

 

I hope this makes sense. This might be a little bit of a fine point, and not necessarily spelled out in the rules, but I think the idea of doing this little point on newly built cars is a good idea - just to make the car that much safer (not to mention stiffer).

[Guest]

I think the reasoning here is that the cross bar IS in shear when the rear downtubes would be in compression. Let me explain.

 

If the car is on it's roof, for instance, and there is a compression loading on the rear supports, then they are pushing down on the shock bar. If the rear tubes were, let's say, attached in the middle of the shock bar, the shock bar would be in bending. As you move those mounts out to the edges, there is less bending load on the shock bar, but the welds at the ends are still in shear - not necessarily a critical flaw, but certainly an opportunity for improvement.

 

When I look at that HANS product above, my eyes are attracted to the large gap between the floor plate and the bottom of the bar. They probably do this so a street car can fit the carpet in between. If the bar were to be lower, say sitting right on top of the horizonatal floor doubler plate and seam welded to it along it's length, then I think you've resolved the issue, since compression loading in the rear down tubes now has a direct load path into the floor doubler panels.

 

I hope this makes sense. This might be a little bit of a fine point, and not necessarily spelled out in the rules, but I think the idea of doing this little point on newly built cars is a good idea - just to make the car that much safer (not to mention stiffer).

 

Scott, with that said when you weld a plate onto the pan and then weld the downtubes to those plates, the plates are then in shear with the pan....anyway you cut it something will be in shear. A large percentage of existing cars use bars like the above one and they are all legal. Why would there be talk of changing this?

 

I think your logic is on the right track but I also think that it's also splitting hairs and very small hairs at that. it's a common practice to mount the downtubes to a rear shock tower bar, why risk eliminating all of those existing cars.

[swhiteh3]

I don't know if this should be ENFORCED, but if it was my car, that's how I'd do it...

[mwilson7]

I don't know if this should be ENFORCED, but if it was my car, that's how I'd do it...

 

Like this:

 

[swhiteh3]

Yup, you can see the mistakes that can be made when you try to build a street car that still has a carpet.

[onralz]

Ok, mine is like the picture, except that I have attached bars that go from the bar extending from tower to tower, to the floor. To me that is the best of both worlds.

[Grizlbits]

Ok, mine is like the picture, except that I have attached bars that go from the bar extending from tower to tower, to the floor. To me that is the best of both worlds.

 

Hey Mark, aren't you the guy who caulks roll bars in? I don't believe design matters much with welds like that.

 

http://www.nasaforums.com/viewtopic.php?t=3314&start=0

[onralz]

I hear you on that. Keeping in mind that I am no welder(but I can weld better than those welds), but those welds look fairly pathetic. They did hold up in turn three at Blackhawk though. Besides, wrong car.

[mwilson7]

I hear you on that. Keeping in mind that I am no welder(but I can weld better than those welds), but those welds look fairly pathetic. They did hold up in turn three at Blackhawk though. Besides, wrong car.

 

You were the chalker!

[onralz]

Huh?

[Guest]

Yup, you can see the mistakes that can be made when you try to build a street car that still has a carpet.

Scott, I don't see any issue with putting the downtubes into the cross bar. It's probably stronger than 1/2 of the other ways I have seen it done. It's really a wash out. As long as the cross bar is tied into the car in a decent manner it will be fine.

 

Let's put this to bed, call it legal and move on.

 

I won't comment on anything to do with Luna, I'm too much of a nice guy!

[Tom Beverly]

Mark, I'm not 100% sure about the reasoning behind the design NASA is pushing. I was told second hand by Lewis what the problem was. Perhaps Adam G. or JWL can clairify the issue at hand. I know they are both aware of the design that was in question.

I don't have the rules in front of me right now either, so I can't look at to see if it is clear or not.

[Lewis Tanner]

OK, here is the interpretation of the rules that I was given that deemed this illegal, as well as the justification that was presented -

 

15.6.13 Mounting Points

The roll cage shall be mounted to the floor of the car in six, seven, or eight points. The

cage shall not go through the firewall. The seventh and eighth points must attach to the

firewall or front fender wells. All cage attachment points must be mounted to plates.

Each required cage bar shall terminate on a plate with a 360 degree weld to the

mounting plate, except as specified in Section 15.6.14.B. There shall be only one (1)

mounting “point” per plate. This point is defined as where the “required tube” mounts.

All additional tubes mounted to that plate must be mounted as close to the required tube

as possible [Ref: (15.6.14.B)].

 

The rear down tubes are required cage bars. Required cage bars must mount to plates. Plates are defined in the following section -

 

15.6.14 Mounting Plates

Each mounting plate shall be no greater than 100 square inches and no greater than 12

inches or less than 2 inches on a side. Welded mounting plates shall be at least 0.080-

inch thick. Plates may extend onto vertical sections of the structure. Any mounting plate

may be multi-angled, but shall not exceed 100 square inches total including vertical

sections. Each mounting plate should have an area of not less than nine (9) square

inches.

 

The above does not qualify a tube as a mounting plate, so mounting a required tube to another tube, regardless of how that tube is attached to the body of the vehicle, does not meet the letter of the rules.

 

Further, mounting to a crossbar that is mounted to the shock towers introduces the following "weak link" issues to the cage-

 

1) The shear joint at the shock tower.

2) The cross bar is in bending which would act on the shear joint in a negative manner: single shear + bending leverage = not so good. This is obviously a function of the distance between the shock tower and the downtube, but still comes into play.

3) The crush strength of the crossbar.

 

I've had a long discussion with more than one NASA official on this, and I'm sure they'll chime soon enough. I can't say that I agree completely with the analysis and reasoning behind the current ruling, as my non-scientific poll suggests that "the other guys" allow this design and this does little to promote cross-series migration of cars and ends up alienating the customers that drive the series for what could be considered a pedantic interpretation of the rules. I'm not particularly happy with the ruling, nor do I believe that those affected by the ruling that already have cars built will be very happy if there isn't a "grandfather" clause or accepted "fix" that allows them to continue running their cars, but that's not my decision to make and that's all the commentary I wish to offer with regards to the situation as I'm not a mechanical engineer and can't address the structural issue beyond what I'd classify as a basic empirical understanding of how these things really work.

 

I'm tired and it's late, so I'm going to bed.

[mwilson7]

I believe that AI rule 15.12 allows rollbar down tubes to be welded to anything that meets the material spec. Mounting plate size and design is unrestricted. Number of points are unrestricted as well (does that mean I could just run with a halo bar!!! )

 

If I had a way to cut and paste I would but all I have is the PDL version of the rules. Could someone post the full text of that rule.

 

If that's not true then most of the AI cars in OH/IN are illegal as well.